Commercial and heavy-duty vehicles in the 14 ton class and higher have been predominantly equipped with drum-type brakes heretofore. In the United States, France and Germany, however, the front wheel brakes of this class of vehicles have been replaced to some extent by compressed air actuated mechanically operated segmental disk brakes but avoid problems which have arisen with drum-type brakes. Municipal busses, for example, have been primarily subject to this type of brake replacement in spite of the fact that the drawback of such segmental disk brakes is that they dissipate the braking heat primarily by radiation, by contrast to drum and full-lining brakes.
With segmental disk brakes, the heat conduction from the disk commences only when a temperature is attained which can significantly cause radiation. This may amount to 400.degree. C. and thus the temperature range at which the segmental brake operates is that which coincides with very high brake-lining wear.
In municipal vehicles involved in city traffic, the brake linings generally can only last for about 10,000 kilometers and must be frequently replaced by comparison with normal highway travel which allows the brake linings to last for 30,000 to 40,000 kilometers.
Drum and full lining brakes can, however, be conveniently used for city travel because they, like clutch linings, show practically no wear because annular brake surfaces have an average temperature range which is below 100.degree. C. in use.
The high wear of segmental disk brakes, releasing aramide fibers nowadays instead of asbestos fibers, nevertheless poses a heavy hazard and can affect the lungs of persons inhaling the fibers.
Thus, while segmental disk brakes have in the past been found to be desirable for heavy-duty vehicles to offset certain aspects of nonreliability of drum-type brakes, they have major disadvantages.
Full lining disk brakes, moreover, have become standard for tractors and tracked vehicles.
Indeed for commercial trucks and busses and heavy duty vehicles generally, a series of full lining disk brakes with mechanical and rod actuation have been proposed. However, direct pressure medium actuation is desirable for providing hysteresis-free antilocking operation.